1. Field of the Invention
This invention relates to a group III-V nitride-based semiconductor substrate and, in particular, to a group III-V nitride-based semiconductor substrate that is suited for a substrate of a nitride-based semiconductor device such as a laser diode (LD) and light emitting diode (LD), and a method of making the same.
2. Description of the Related Art
Group III-V nitride-based semiconductor materials such as gallium nitride (GaN) have a wide bandgap and are of direct transition type. Therefore, they attract attention as a material for ultraviolet to blue light emitting device.
Thus far, a hetero-epitaxial substrate such as a sapphire substrate is used to make a GaN-based semiconductor light emitting device. However, it is not possible to grow a GaN single crystal film even directly on the sapphire substrate since the sapphire substrate has a lattice constant different from GaN.
JP-A-4-297023 discloses a method that an AlN or GaN buffer layer is in advance grown on a sapphire substrate at a low temperature, thereby reducing the lattice distortion, and then GaN is grown on the buffer layer. With such a low temperature growth buffer layer, it becomes possible to obtain an epitaxially grown single-crystal GaN. However, even in this method, lattice mismatch between the sapphire substrate and the grown crystal cannot be eliminated and GaN thus obtained has a number of defects.
In recent years, ELO (e.g., OK-Hyun Nam et al., “Lateral Epitaxy of Low Defect Density GaN Layers via Organometallic Vapor Phase Epitaxy”, Appl. Phys. Lett. 71 (18) p 2638 (1997)) and FIELO (e.g., Akira Usui et al., “Thick GaN Epitaxial Growth with Low Dislocation Density by Hydride Vapor Phase Epitaxy”, Jpn. J. Appl. Phys. Vol. 36, pp. L899-902 (1997)) are reported that are methods for reducing a defect density caused by lattice mismatch between sapphire and GaN. In these methods, a mask material such as SiO2 is by photolithography formed partially on an underlying GaN single crystal film which is grown on a sapphire substrate by MOVPE (metalorganic vapor phase epitaxy), and GaN is subsequently grown thereon. Thereby, the propagation of dislocation from the underlying layer can be suppressed. Further, the high-quality GaN thick film thus obtained is separated by laser separation or etching etc. to have a GaN self-standing substrate.
However, the GaN self-standing substrate thus made is warped into a concave form in as-grown state. Such a warping is essential in the GaN growth method using the heteroepitaxy of Volmer-Weber growth mode. For example, when GaN is grown on a hetero-substrate such as sapphire, microscopic GaN islands are densely formed at the initial step of growth, and then they are enlarged according to the growth, combined with each other, finally forming a flattened plane, and transferred into two-dimensional growth. The islands attract each other to minimize the surface energy when being combined, and a tensile stress is generated thereby. Further, even after the combination, contraction in volume occurs due to elimination of grain boundary along with the growth. It is assumed that these cause the concave warping. Although the warping can be lessened to some degree by control at the initial step of the growth, it is difficult to reduce the warping to zero thereby.
It is found by the inventors that a kind of doping in the growth of GaN functions to increase the warping.
FIG. 14 is a graph showing the relationship between a Si doping concentration and a warping of GaN substrate when Si is uniformly doped into GaN. In view of this graph, it is found that the warping increases as Si concentration in GaN crystal increases.
FIG. 15 is a graph showing the relationship between a thickness and a warping of doping layer when its doping concentration is set to be constant (5×1018 cm−3) and the doping layer is formed on its upper surface side. In view of this graph, it is found that the warping increases as the thickness of the doping layer increases.
FIG. 16 is a graph showing the relationship between a doping start position and a warping of doping layer when Si is doped at a concentration of 5×1018 cm−3 and the doping layer has a thickness of 100 μm. In view of this graph, it is found that the warping depends on the doping start position in thickness direction, and the warping rapidly increases when the doping starts from a position near the seed substrate.
The correlation between the doping and the warping as shown in FIGS. 14 to 16 is assumed to be caused by a change in lattice constant due to Ga or N site being replaced by an element with a different atomic radius. Thus, although the doping is needed to provide a necessary conductivity for the GaN substrate, it can cause a further increase of the warping.
As described, due to the GaN growth mechanism and the doping, the GaN self-standing substrate is warped into the concave form in as-grown state.
Even when both surfaces of the substrate being concave-warped in as-grown state are apparently polished to be flattened, the substrate must have a distribution in crystal orientation since the outer shape thereof is only corrected.
Therefore, when a light emitting device is fabricated on such a substrate, the emission wavelength will be distributed according to the orientation distribution of the substrate. This is assumed because the step density varies due to the difference of surface orientation and, therefore, amount of In taken in varies when an active layer of InGaN etc. is grown thereon. This causes a decrease in device yield eventually.